It is known that addition of certain kinds of cyanine dyes is highly effective in enhancing the sensitivity of silver halide photographic emulsions. In such a case, the cyanine dye added to the silver halide photographic emulsion is adsorbed by the silver particles in the emulsion to add an absorption band of greater wavelength to the absorption band which is inherent in the silver halide. This phenomenon is called "spectral sensitization". It is also known that the efficiency of color sensitization by this dye may be notably enhanced when the dye is used in the presence of a second, certain kind of specifically selected dye or other organic substance and that a selective combination of certain types of dyes provides superadditively high sensitivity. This effect is called "supersensitization". Generally, combined use of two or more dyes more often than not fails to improve sensitivity or even results in loss of sensitivity. Therefore, supersensitization is a specific phenomenon depending on very specific combinations of compounds.
The maximum luminosity (maximum visibility) of man is about 545 nm. From the fact that the human eyes are acutely sensitive to light in the green wavelength region, it is inferred that the technique for sensitizing the green wavelength region is particularly significant in the whole technique for spectral sensitization. The term "luminosity" in a certain wavelength means a reciprocal of quantity of radiation having the wavelength required for providing a certain lightness.
Addition of a certain type of benzimidazolocarbocyanine dye is an unusually effective means of enhancing the sensitivity of silver halide to green light. This fact is described, e.g., in U.S. Pat. Nos. 2,739,149 and 2,912,329 and in British Pat. Nos. 654,690 and 815,172. The conventional benzimidazolocarbocyanine dye often fails to provide sufficient sensitization when it is used by itself. Several techniques involving combined use of the benzimidazolocarbocyanine dye with certain other dye with a view to the effect of supersensitization have been reported to the art. Descriptions dealing with such techniques are found, for example, in U.S. Pat. No. 3,506,443, Japanese Patent Publication No. 4936/68 and Japanese Patent Application (OPI) No. 82416/77 (the term "OPI" as used herein refers to a "published unexamined Japanese patent application").
Supersensitized emulsions using the conventionally known benzimidazolocarbocyanine dye, however, have suffered aggravated fog of the applied emulsion upon exposure to high temperature or to the combination of high temperature and high humidity or degraded stability of the applied emulsion to withstand effects of aging, with resultant extensive loss of sensitivity of the emulsion.
Improvements directed to overcoming the drawbacks suffered by supersensitized emulsions using the benzimidazolocarbocianine dye, namely, the aggravated fog of the applied emulsion under the influence of high temperature or the combination of high temperature and high humidity and the gradual degradation of sensitivity of the emulsion under the influence of aging, therefore, have constituted one of the important problems to be solved in techniques for the production of lightsensitive materials.